In recent years, as a communication system providing an information service with high speed and large capacity, a WDM (Wavelength Division Multiplexing) optical communication system has been developed. In particular, an optical communication system using an arrayed waveguide diffraction grating as an optical wavelength multiplexer/demultiplexer, in which plural optical signals with different wavelengths can be multiplexed or demultiplexed, is greatly expected. For example, this type of optical communication systems using an arrayed waveguide diffraction grating are disclosed in Japanese Patent Application Laid-Open Nos. 4-116607,4-163406, 4-220602, 4-326308, 5-157920, etc.
In the conventional arrayed waveguide diffraction grating type optical wavelength multiplexer/demultiplexer, when a pass band characteristic is flattened, the fluctuation of an optical insertion loss caused by the wavelength fluctuation of a light source is relatively small, so that optical signals can be multiplexed and/or demultiplexed in a static state. Accordingly, the arrayed waveguide diffraction grating type optical wavelength multiplexer/demultiplexer is especially expected as an effective device for the optical WDM communication. This advantageous feature of the optical wavelength multiplexer/demultiplexer is described, for example, in U.S. Pat. No. 5,412,744.
As a structure of an optical wavelength multiplexer/demultiplexer using an arrayed waveguide diffraction grating, a structure comprising an input waveguide and output waveguides which are coupled to both ends of an arrayed waveguide diffraction grating via an input slub waveguide and an output slab waveguide, respectively, has been known.
FIG. 1 shows a structure of a conventional optical wavelength multiplexer/demultiplexer of this type, wherein the optical wavelength multiplexer/demultiplexer comprises a substrate 1, an input waveguide 2 formed on one side of the substrate 1, output waveguides 3 formed on another side of the substrate 1, and an arrayed waveguide diffraction grating 4 formed on a central portion of the substrate 1, wherein the arrayed waveguide diffraction grating 4 is composed of a plurality of channel waveguides 5 in parallel having predetermined path lengths.
The optical wavelength multiplexer/demultiplexer further comprises an input slub waveguide 6 for coupling the input waveguide 2 and the arrayed waveguide diffraction grating 4, an output slub waveguide 7 for coupling the output waveguides 3 and the arrayed waveguide diffraction grating 4, and a mode conversion portion 8 provided between the input waveguide 2 and the input slab waveguide 6 for flattening the optical loss-wavelength characteristics.
For example, in this conventional optical wavelength multiplexer/demultiplexer, for demultiplexing multiplexed optical signals, multiplexed optical signals .lambda..sub.1 to .lambda..sub.9 each having different wavelengths, wherein .lambda..sub.1 to .lambda..sub.9 indicate different wavelengths and the relation .lambda..sub.1 &lt;.lambda..sub.2 &lt; - - - &lt;.lambda..sub.8 &lt;.lambda..sub.9 is set, are input from the input waveguide 2 and transmitted through the mode conversion portion 8, then radiated into the input slab waveguide 6. Next, the multiplexed optical signals .lambda..sub.1 to .lambda..sub.9 are divided at an input end 9 of the arrayed waveguide diffraction grating 4, then transmitted through the channel waveguides 5 and an output end 10 of the arrayed waveguide diffraction grating 4, and focused at a focusing plane 11 of the output slab waveguide 7. Thus, the multiplexed optical signals .lambda..sub.1 to .lambda..sub.9 are demultiplexed and output from the output waveguides 3 having nine ends as demultiplexed optical signals .lambda..sub.1, .lambda..sub.2 - - - .lambda..sub.9, respectively.
According to the conventional optical waveguide multiplexer/demultiplexer, however, the optical characteristics required for practical use cannot be obtained sufficiently. For example, when the optical characteristics of the device is influenced by the stigmatism of the output slub waveguides, etc., the flatness of the optical insertion loss in the pass band becomes insufficient. Accordingly, if the wavelength shows a slight fluctuation, the optical insertion loss characteristic of the waveguide will be greatly fluctuated.
Next, thisphenomenon will be explained referring to FIGS. 2A, 2B, and 2C to 5.
FIGS. 2A to 2C show electric field distributions at the mode conversion portion 8 in the direction A-A', the input end 9 of the arrayed waveguide diffraction grating 4 in the direction B-B', and the output end 10 in the direction C-C', respectively. At the mode conversion portion 8, an electric field distribution 13' has a twin-peaks-shape profile. At the input end 9, because of the diffraction effect, an electric field distribution 14, has a maximum peak L' and minimum peaks m.sub.1 ' and m.sub.2 '. At the output end 10, an electric field distribution 15' reprises a profile of the electric field distribution 14' at the input end 9.
FIGS. 3A to 3C show phase distributions of the optical signals .lambda..sub.1, .lambda..sub.5, and .lambda..sub.9 at the output end 10 of the arrayed waveguide diffraction grating 4, respectively. A phase distribution 17' of the optical signal .lambda..sub.5 shows a symmetric phase profile as shown in FIG. 3B. On the other hand, a phase distribution 16' of the optical signal .lambda..sub.1 and a phase distribution 18' of the optical signal .lambda..sub.9 show asymmetric phase profiles as shown in FIGS. 3A and 3C, respectively.
It is because that the optical wavelength multiplexer/demultiplexer is designed based on a propagation constant of an intermediate wavelength .lambda..sub.5. Namely, comparing with the phase profile of the optical signal .lambda..sub.5, the phase profiles of the optical signals .lambda..sub.1 and .lambda..sub.9 are inclined to the arrayed waveguide diffraction grating 4 in accordance with the respective propagation constants thereof.
Accordingly, a phase profile 19' of FIG. 3D, which is a phase difference between the phase distributions 16' and 17' shown in FIGS. 3A and 3B, shows a continued inclination in the right upper direction. On the other hand, a phase profile 21' of FIG. 3E, which is a phase difference between the phase distributions 17' and 18' shown in FIGS. 3B and 3C, shows a continued inclination in the left upper direction which is opposite to that of FIG. 3D.
A length difference .DELTA..sub.L between two adjacent channel waveguides 5 based on the propagation constant of the intermediate wavelength .lambda..sub.5 is determined by a following formula (1): EQU .DELTA..sub.L =2.multidot.m.multidot..pi./.beta.(.lambda..sub.5) (1)
wherein, m is a diffraction order number (a positive integer) and .beta.(.lambda..sub.5) is a propagation constant of the channel waveguides for the optical signal .lambda..sub.5.
FIG. 4 shows an electric field distribution at the focusing plane 11 in the direction D-D'.
As shown in FIG. 4, an electric field distribution 24' at a position x.sub.5 corresponding to the optical signal .lambda..sub.5 has a symmetric twin-peaks-shape profile, similarly to the electric field distribution at the mode conversion portion 8. On the other hand, an electric field distribution 23' of the optical signals .lambda..sub.1 at a terminal position x.sub.1 and an electric field distribution of the optical signal .lambda..sub.9 at a terminal position x.sub.9 have asymmetric profiles, respectively, because of the stigmatism of the output slab waveguide 7.
The optical insertion loss of the output waveguides 3 are determined by a multiplexed integral of an electric field distribution of optical signals at the focusing plane 11 and an inherent mode of the output waveguides 3. However, it is evidently undesirable that the electric field distribution becomes more asymmetric in accordance with the increase of the distance from the central position x.sub.5 as described above.
FIG. 5 shows an optical loss to wavelength characteristics, namely a relationship between an optical loss and respective wavelengths of demultiplexed optical signals .lambda..sub.1 to .lambda..sub.9. As shown in FIG. 5, since a pass band corresponding to the central position x.sub.5 has a flat characteristic shown as a profile 27', even if the wavelength .lambda..sub.5 is slightly fluctuated, the fluctuation of the optical insertion loss is not caused. On the other hand, the pass bands corresponding to the positions x.sub.1 and x.sub.9, where the electric field distributions thereof are asymmetric, have inclined characteristics shown as profiles 26' and 28', respectively. As a result, the optical insertion loss thereat is fluctuated when the wavelength of the optical signals is slightly fluctuated at the light source.